Multi-layered information recording media are known as media capable of storing a large number of information items. Currently, double-layered DVD-ROM (video format) and double-layered DVD+R are standardized and put into practical use as multi-layered recording information media.
FIG. 1 illustrates a general structure of a double-layered DVD+R. The double-layered DVD+R structure includes a first substrate 1, a semi-transmissive layer (L0) 2, an intermediate layer 3, a reflective metal film (L1) 4, and a second substrate 5, arranged in this order from the light incident surface.
In FIG. 1, information signals are recorded as patterns of specific shapes in the surface of the semi-transmissive layer 2 or the reflective metal film 4. The first substrate 1 and the second substrate 5 are generally made of polycarbonate. The intermediate layer 3 is generally made of an ultraviolet-curable or thermal-curable resin. The semi-transmissive layer 2 is made of, for example, silicon, silver, or aluminum. The reflective metal film 4 is made of silver or aluminum as a major constituent.
Because the disk thickness (the distance from the light incident surface to the recording layer) is different between a single-layer DVD+R and a double-layered DVD+R, both of which enable a user to read and write data in the disks, spherical aberration is generated. In addition, because the track pitch is the same in the respective recording layers in the double-layered information recording medium illustrated in FIG. 2A and in the single-layer information recording medium, data recording quality and data reproduction quality are degraded when the same optical system is used without aberration correction means.
To overcome the latter problem, optical systems with spherical aberration correcting elements have been developed in order to eliminate the variation in the recording quality and the data reproduction quality between the single-layer information recording media and double-layered information recording media. For example, Patent-related Publication 1 listed below discloses a technique for correcting spherical aberration by driving lenses other than the objective lens to change the optical magnification incident on the objective lens. Patent-related Publication 2 discloses a technique for correcting spherical aberration by shifting the phase of the light beam incident on the objective lens making use of the difference in refractive index of liquid crystal.
Patent-related Publications 3-5 listed below disclose a technique for improving the initial accessing speed by allowing the spherical aberration correcting element to be perceived at the same position between a single-layer information recording medium and a double-layered information recording medium when these information recording media are set in a recording apparatus using the above-described spherical aberration correcting techniques.
All of these known techniques are based upon the idea of making the recording layer of an information recording medium be consistent with the location of the recording layer of either a single-layer information recording medium or a double-layered information recording medium.
In recent years and continuing, there is a strong demand for further improvement in the reading and writing speed with respect to information recording media. It is also required for information recording media to have a structure for allowing high-speed access not only in the initial access speed, but also in the overall read/write speed. One method for reducing the read/write speed is to reduce the time required for inter-layer jumping (for allowing the focal spot of the light beam to jump between recording layers) during the reading and/or writing operations.
With the conventional techniques, inter-layer jump requires longer time because the spherical aberration correcting element has to be moved along with the shift of the focal spot of the objective lens. To overcome this problem, Patent-related Publication 6 proposes not to use a spherical aberration correcting element, and instead, it proposes to change the information items (including the track pitch and the minimum mark length) recorded in each of the recording layers linearly with respect to the disk thickness of that recording layer, based on the relationship between the degradation of the spot size due to the difference in the disk thickness, and the information density relative to the spot size, in order to enable high-speed access and to maintain the signal quality the same in the respective recording layers.
<Publications Referred to in the Specification>
                Patent-related Publication 1: JP 05-266511 A        Patent-related Publication 2: JP 2,895,150 B        Patent-related Publication 3: JP 2002-352469 A        Patent-related Publication 4: JP 2003-281779 A        Patent-related Publication 5: JP 2003-346379 A        Patent-related Publication 6: JP 11-16207 A        Patent-related Publication 7: JP 2003-91822 A        